In the production of paper a surface gloss is produced depending on the type of paper; for example, magazine or newsprint. In the paper production process calendering rolls impart a gloss to the surface of the moving paper web. The surface of the calender roll is a hard cylindrical material (steel) and by the use of heat and pressure, the range of gloss development is controlled.
To supply heat to the calender roll or the associated paper web two techniques have been used; direct steam applied to the paper web (for example, see U.S. Pat. No. 4,786,529) or internal heating of the steel calender roll itself by, for example, by inductive coils or the flow of hot oil. By control of such heating, the latex coating on the paper is plasticized and by varying such plasticizing the degree of gloss development is controlled.
In general terms, the surface of the polished metal calender roll is replicated. In the above processes, the application of steam creates too much moisture which affects other parameters of the paper, such as thickness, basic weight, moisture content, etc. The oil or inductive coil technique is very slow. Also in the stream of the paper making process where a super calender roll is used (this is to promote and increase the range of gloss development), normally the steel rolls, which may be 8 or 10 stacked on top of each other commingled with pressure and idler rolls, do not have internal access.
The above '529 patent points up out the problem of inaccurate cross direction control, thus causing a non-uniform cross direction gloss profile. However, another more major problem is that since a portion of the calender roll is covered by the paper, the web in effect cools the contacted portion of the calender roll more than the uncovered ends. The calender roll itself has unequal expansion or contraction of its diameter to produce an hour-glass type shape. In other words, due to the cooling effect of the paper web, the center portion of the roll which is covered by the paper web is of a lesser diameter. This means that the pressure of the adjacent roll (for example, made of artificial material such as NOMEX.TM.) is less in the the center than towards the ends of the calender roll. Unwanted changes both in gloss and thickness (caliper) may also occur. In other words, the greater diameter of the outer edges increases the pressure and calendering effect.